A variety of semiconductor elements, such as a memory unit, a CMOS, and a CPU, that are manufactured in the wafer process have terminals for electrical connection. The terminals for electrical connection are different in pitch from the connectors of the printed wiring board to which the semiconductor element is to be attached by a factor of approximately several times to hundreds of times. Therefore, in the case of connecting a semiconductor element to a printed wiring board, an intermediate substrate for converting a pitch (a substrate for packaging a semiconductor element), which is called an interposer, is used.
A semiconductor element is packaged on one surface of the interposer. Connection with a printed wiring board is obtained on the other surface or around the interposer. The interposer has a metal lead frame in its inside or on its surface. The lead frame is used to route electrical connection paths, to thereby expand the pitch of external connection terminals for connection with the printed wiring board.
FIG. 2A to FIG. 2C show a schematic structure of a lead frame substrate of QFN (Quad Flat Non-Lead) type as an example of an interposer.
As shown in FIG. 2A, on a central portion of a lead frame made of aluminum or copper, there is provided a flat portion 21 on which a semiconductor element 22 is arranged. Around a perimeter of the lead frame, leads 23 with a wide pitch are disposed. The wire bonding method by metal wires 24 such as gold wires is applied to connect between the leads 23 and the terminals for electrical connection of the semiconductor element 22. As shown in FIG. 2B, the constituents are finally molded with a molding resin 25 into integration.
A retaining member 27 in FIG. 2A and FIG. 2B is for retaining the lead frame. The retaining member 27 is removed after molding with the molding resin 25, as shown in FIG. 2C.
With the interposer shown in FIG. 2A to 2C, electrical connection is obtained only between the perimeter of the semiconductor element 22 and the leads 23 around the perimeter portion of the lead frame. Therefore, the interposer is unsuitable for a semiconductor element with a multitude of terminals.
In the case of a small number of terminals, connection between the printed wiring board and the interposer is obtained by metal pins being attached to extraction electrodes 26 around the perimeter portion of the interposer. In the case of a large number of terminals, a BGA (Ball Grid Array) is used in which solder balls are arranged in an array on external connection terminals around the perimeter portion of the interposer.
For a semiconductor element with a small area and a large number of terminals, pitch conversion is difficult with an interposer with only a single wiring layer. To address this, a technique of making the wiring layer into a plurality of wiring layers and stacking them is adopted.
In the case of a semiconductor element with a small area and a large number of terminals, the connection terminals of the semiconductor element are often formed in an array on a bottom surface of the semiconductor element. Therefore, the external connection terminals of the interposer are also arranged in an array. For connection between the interposer and the printed wiring board, a flip chip connection system using microscopic solder balls is adopted. For wiring inside the interposer, holes are bored in the vertical direction from above with a drill or by laser, and the internal surfaces of the holes are subjected to metal plating. Thereby, electrical conduction is obtained between the top and bottom of the interposer.
In the interposers according to the above system, it is possible for the pitch of the external connection terminals to be microsized to approximately 150 to 200 μm. Consequently, the connection terminals can be increased in number. However, reliability and stability of the joint are decreased. Therefore, the interposers are not suitable for application as in-vehicle interposers that require high reliability.
According to the materials and structures that are used, these interposers are divided into several types including one in which the structure for supporting the lead frame portion is made of ceramic and one in which the base material is made from organic matter such as a P-BGA (Plastic Ball Grid Array), a CSP (Chip Size Package), or an LGA (Land Grid Array). An appropriate one is used according to the purpose and the application.
In any of the aforementioned cases, there are advances in finer pitch and readiness to high-speed signals of the connection portions of a interposer with a semiconductor element as the semiconductor element is decreased in size and increased in the number of pins and in speed. In consideration of development in microsizing, the terminal portion is required to have a pitch of 80 to 100 μm.
The lead frame functioning as both of a conductive portion and a supporting member is formed by etching a thin metal. For a stable etching treatment, and handling for the subsequent working, it is desirable that the metal plate have a thickness of approximately 120 μm. Furthermore, to obtain sufficient bonding strength at the time of wire bonding, a metal layer thickness and a land area to some degree are required. In consideration of these two points, least requirement for the thickness of the metal plate is estimated to be approximately 100 to 120 μm. In this case, the leads are made finer, at most, to approximately 120 μm for the pitch and approximately 60 μm for the line width when the metal plate is etched from both sides.
In addition, another problem is that, in the process of manufacturing a lead frame, a retaining member is discarded after the retaining member is used. This leads to an increase in costs.
This will be further described with reference to FIG. 2A to 2C.
A lead frame is attached to a retaining member 27 made of a polyimide tape. On a flat portion 21 of the lead frame, semiconductor elements 22 are fixed with a fixation resin 28 or a fixation tape 28. After that, wire bonding is performed. Then, the transfer mold method is used to mold the semiconductor elements 22 in a batch with a molding resin 25. Finally, the entire unit is subjected to encapsulation work, and then cut into each semiconductor element 22. The edges of the semiconductor elements 22 are trimmed into completed products. In the case where the rear surface 29 of the lead frame is used as a connection surface with a printed wiring board, the retaining member 27 is indispensable to prevent the molding resin 25 from flowing onto the connection terminal surfaces of the rear surfaces of the leads 23 and attaching there at the time of molding.
However, the retaining member 27 becomes finally unnecessary. Therefore, the retaining member 27 is removed and discarded after molding. This leads to an increase in costs.
For example, Patent Document 1 as an invention in this field has already disclosed an invention that provides a semiconductor apparatus with built-in components by packaging a semiconductor element and a passive element simultaneously on the same lead frame and molding them.
Furthermore, as one solution to solve the aforementioned problem and provide a lead frame substrate that allows formation of traces at a super fine pitch, allows stable wire-bonding work, and is excellent in economic efficiency (although not disclosed in Patent Document 1), for example a lead frame substrate with a pre-molding resin as a supporting member of traces can be conceived.
In this solution, for manufacturing a lead frame substrate, a first resist pattern for forming connection posts is formed on a first surface of a metal plate, and a second resist pattern for forming a wiring pattern is formed on a second surface. Copper of the first surface is etched to a desired thickness from above, and then a pre-molding resin is coated on the first surface to form a pre-mold layer. After that, the second surface is etched to form traces. Subsequently, the first and second resist patterns on both surfaces are stripped.
It is assumed that the following merits can be expected from the lead frame substrate manufactured in this manner. That is, even if the thickness of the metal is made thinner to a level that allows fine etching, it is possible to perform stable etching because the pre-molding resin functions as a support member. Moreover, because of small scattering of ultrasonic wave energy, the lead frame substrate is excellent in wire bondability. In addition, the retaining member 27 made of a polyimide tape is not used. Therefore, it is possible to suppress an increase in costs resulting from the use of the retaining member 27.